The communications industry is rapidly changing to adjust to emerging technologies and ever increasing customer demand. This customer demand for new applications and increased performance of existing applications is driving communications network and system providers to employ networks and systems having greater speed and capacity (e.g., greater bandwidth). In trying to achieve these goals, a common approach taken by many communications providers is to use packet switching technology. Increasingly, public and private communications networks are being built and expanded using various packet technologies, such as Internet Protocol (IP).
Resilient Packet Ring (RPR) networks are based on two counter-rotating rings that transfer packets between nodes. The rings are generally referred to as inner and outer. RPR nodes contain transit buffers to hold traffic that is transiting through the node. The emerging standard allows for one or two transit buffers; the second transit buffer tends to be very large, on the order of hundreds of kilobytes or even megabytes.
RPR networks can use a steering based protection mechanism in order to restore traffic flow following a facility or nodal failure. Steering based protection refers to the fact that that source stations will change the direction of transmission if the destination station is no longer reachable due to the failure.
In order to support efficient bridging, packets may be flooded by a source on both ring; using a time-to-live (TTL) mechanism to limit their travel on the rings such that they do not overlap. However, if a node is optically bypassed (or in a mode where it behaves like a optical pass-through), the TTL mechanism will fail and the packet will be double delivered to one node. Eventually, the topology mechanism will determine the bypass exists and will modify the flooding scope. However, if the preferred ring to deliver packets is changed, then there is a synchronization race to determine when to start accepting the packets from the other ring.
Consider the following scenario that illustrates another mechanism for misordering. Traffic is flowing on the outer ring between the source and destination. Immediately adjacent to the source, the outer ring is broken. Therefore, some packets are in flight from the source to the destination on the outer ring just past the break. The source station learns of the failure and steers traffic to the inner ring. Now, with the appropriate traffic pattern on the ring the transit buffers on the outer ring could be very congested, while the inner ring is uncongested. Therefore the newly steered traffic could arrive at the destination prior to the traffic on the outer ring.
One method of insuring that reorder does not occur would be to wait to steer the traffic, until all of the traffic on the outer ring is delivered. However, due to the size of the transit buffers this could take a large amount of time. As RPR rings are supposed to protect in under fifty milliseconds. It is clear that steering cannot be guaranteed to protect in under fifty milliseconds and insure no misorder. New methods and apparatus are desired which may reduce or eliminate packet reorder and/or duplication.